Process for the production of a diamond heat sink

ABSTRACT

A diamond heat sink having a very high thermal property, to be used for radiation of semiconductor devices or compressors, includes a plate-shaped diamond substrate and fins for increasing the thermal property. The fins are combined with the substrate and are of a material having a heat conductivity of at least 1 (W/cm·K), for example, diamond. Such heat sink is produced by a simple process including arranging a base material and fins for growing diamond in such a manner that the surface of the base material and the upper ends of the fins are substantially the same height by the use of a suitable supporting member or by working the base material itself and growing diamond thereon by a gaseous phase synthesis method.

This is a divisional application of Ser. No. 08/257,288, filed Jun. 9,1994, now U.S. Pat. No. 5,642,779.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for forming a heat sink, inparticular a diamond heat sink having a very high thermal property, usedfor radiation of semiconductor devices or compressors.

2. Description of the Prior Art

With the miniaturization and increase of processing speed of informationprocessing systems, the processing capacity per unit area of asemiconductor device in such a system is rapidly increasing. Thisresults in an increase of the heat quantity of heat per unit areagenerated in the semiconductor device, and the importance of maintainingthe thermal property thereof has been observed with keen interest whendesigning a substrate to be mounted. Up to the present time, a number ofcombinations of alumina substrates and metallic heat sinks with finshave been used, but because of the low heat conductivity (0.29 W/cm·K)of alumina, a sufficient thermal property cannot be obtained, even incombination with the metallic heat sink.

Requirements for the thermal property of a package to be mounted bysemiconductor devices have become more sever because of an increase ofthe heating value with the high effectiveness of the devices to bemounted. Thus, as a means for lowering the heat resistance of thepackage, for example, it has hitherto been known to employ a higher heatconductivity material or to improve convection heat conduction by forcedair cooling, forced water cooling, etc.

As a heat sink material of a semiconductor device having a higherthermal property, it has been proposed to use diamond, cubic boronnitride (CBN), aluminum nitride, etc. However, these high heatconductivity materials have a problem that, though their heatconductivity is higher than that of alumina, etc., their production costis higher. In particular, diamond is contemplated as a heat sink forsemiconductors, since the gaseous phase synthesis technique has latelybeen developed and has progressed, and a sheet-shaped heat sink with alarge area can be obtained. However diamond is much more expensive thanthe ordinary materials of the prior art and its high heat conductivitycannot effectively be utilized, since a fin-fitted diamond heat sinkcannot be prepared and joint use of a metallic fin-fitted heat sink isrequired.

Ordinary materials used for the package, for example alumina, willsurely meet with a problem regarding the thermal property thereof when ahigher performance device is put to practical use in the near future.Namely, in the present package, there will arise a problem that the heatresistance thereof is so large that heat generation of the device itselfcannot be radiated sufficiently, thus raising the temperature of thedevice and the device will fail to function normally. In order to solvethis problem, it is effective to use a high heat conductivity material,as described above, and diamond having the highest heat conductivity ofprevailing materials has been used as a semiconductor laser diode. Atthe present time, as a diamond for radiation, there are used naturalsingle crystal diamond or artificial single crystal diamond prepared bya high pressure synthesis method and polycrystal prepared by a gaseousphase synthesis method. However they generally are limited toplate-shaped crystals. However, the heat transferred by a heat sinkshould finally be discharged through a fluid such as air or coolingwater, and a structure with a large radiation area, such as a fin-fittedheat sink, is preferable for improving the heat exchange function withsuch a fluid.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a heat sink of smallsize, having a very high thermal property, and capable of being used asa heat sink for mounting semiconductor devices.

It is another object of the present invention to provide a diamond heatsink having a very high thermal property and used for radiation ofsemiconductor devices or compressors.

It is a further object of the present invention to provide a process forthe production of a diamond heat sink having a very high thermalproperty.

These objects are achieved by forming a heat sink comprising aplate-shaped substrate and fins to increase the thermal property, thefins being combined with the substrate and being of a material having aheat conductivity of at least 1 (W/cm·K), preferably at least 5(W/cm·K).

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings illustrate the principle and merits of theinvention in greater detail.

FIG. 1 is a plan view of a heat sink formed according to one embodimentof the present invention.

FIG. 2 is an end view of a heat sink part of a ceramic PGA (Pin GridArray) package employed in the heat sink of FIG. 1.

FIG. 3 is a sectional view of the heat sink part shown in FIG. 2, inwhich hollows or grooves are formed.

FIG. 4 is a sectional view of the heat sink of FIG. 1.

FIGS. 5(a)-5(d) are sectional views illustrating another embodiment of aprocess for the production of a heat sink according to the presentinvention.

FIGS. 6(a)-6(c) are sectional views illustrating a further embodiment ofa process for the production of a heat sink according to the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The inventors have made detailed studies on the structure of a heat sinkused for mounting semiconductor devices in order to solve the abovedescribed problems, and consequently have found that even when anordinary material such as alumina is used for a heat sink, the thermalproperty is sufficiently improved by making only fin parts of the heatsink of a high heat conductivity material such as diamond or cubic boronnitride. Of course, when a substrate or base also is made of a high heatconductivity material such as diamond, a higher effect can be obtained.Furthermore, it has been found that the thermal property is furtherimproved by burying fins formed of a high heat conductivity material ina base material to be near a side thereof where semiconductor devicesare mounted on the base material, as opposed to bonding the fins to thesurface of the base material.

Thus, heat resistance can be decreased substantially by forming a heatsink having a fin structure of a large heat conductivity material on theradiation surface. Namely, the present invention provides a method ofproducing a heat sink comprising a plate-shaped substrate and fins forincreasing the thermal property, the fins being combined with or joinedto the substrate and being of a material having a heat conductivity ofat least 1 (W/cm·K), preferably at least 5 (W/cm·K).

According to the present invention, there is provided a method ofproducing a diamond heat sink comprising at least a plate-shaped diamondsubstrate and fins provided on the diamond substrate to increase thethermal property, the fins being of a material having a heatconductivity of at least 1 (W/cm·K), and more specifically, there isprovided a diamond heat sink comprising at least a plate-shaped diamondsubstrate and fins provided on the diamond substrate to increase thethermal property and formed of diamond.

The present invention provides a very simple process for the productionof a diamond heat sink having a fin structure by a gaseous phasesynthesis method. Such process includes arranging a base material andfins for growing diamond in such a manner that a surface of the basematerial and an upper end of the fin are substantially the same heightby the use of a suitable supporting member or by working the basematerial itself and growing diamond thereon by the gaseous phasesynthesis method. The fin material is not required to be free-standingand can be a material having a heat conductivity of at least 1 W/cm·K. Afree-standing film is a film having a thickness capable of substantiallymaintaining a shape as a diamond film even if the base material isremoved, and ordinarily having a thickness of about at least 0.1 mm.

For example, there are processes comprising arranging suitable blockswith gaps therebetween for forming fins, inserting diamond free-standingfilms in the gaps, placing a base material for the growth of diamond onthe blocks and growing diamond by gaseous phase thereon, or comprisingforming grooves for inserting diamond free-standing film previouslyprepared in a desired size on a substrate which can readily be worked,inserting the diamond free-standing films in the grooves, then growingdiamond thereon and removing the base material.

According to the present invention, one process for the production of adiamond heat sink having fins for increasing the thermal propertyincludes a step of working or arranging at least a base material in sucha manner that the base material is provided with gaps for inserting finsand the highest parts of the fins when inserted and the surface of thebase material are substantially the same height, a step of inserting thefins in the gaps, a step of growing diamond on the base material and thefins, and a step of removing the base material to obtain a diamond heatsink having the fins.

Another process for the production of a diamond heat sink having finsfor increasing the thermal property includes a step of cutting to asuitable shape at least a diamond plate prepared by a gaseous phasesynthesis method to obtain divided diamond base materials, a step ofplacing the diamond base materials on a supporting member in such amanner that there are formed gaps for inserting fins, with the highestpart of the fins when inserted and the surface of the divided diamondbase materials being substantially the same height, a step of insertingthe fins in the gaps, and a step of growing diamond on the diamonddivided base materials and fins, thus obtaining a diamond heat sinksubstrate having the fins without removing the diamond divided basematerials.

Another process for the production of a diamond heat sink having finsfor increasing the thermal property includes a step of arranging dividedbase materials on blocks with gaps therebetween for inserting fins, astep of inserting fins in the gaps of the divided base materials in sucha manner that the highest parts of the fins and the surface of thedivided base materials are substantially the same height, a step ofgrowing diamond on the divided base materials and fins, and a step ofremoving the divided base material to obtain a diamond heat sink havingthe fins.

Another process for the production of a diamond heat sink having finsfor increasing the thermal property includes a step of cutting a diamondplate prepared by a gaseous phase synthesis method in a suitable shapeto obtain divided base materials, a step of arranging the divideddiamond base materials on blocks with gaps therebetween for insertingfins, a step of inserting the fins in the gaps of the divided diamondbase materials in such a manner that the highest parts of the fins andthe surface of the divided base materials are substantially the sameheight, and a step of growing diamond on the divided diamond basematerials and fins, thus obtaining a diamond heat sink having the finswithout removing the divided diamond base materials.

Another process for the production of a diamond heat sink having finsfor increasing the radiation property includes a step of forminggrooves, with a suitable depth for inserting fins, in a base materialfor growing at least diamond, a step of inserting fins having a heightsubstantially the same as the depth of the grooves in the grooves formedin the base material, a step of growing diamond by a gaseous phasesynthesis method on the base material in which the fins have beeninserted, and a step of removing the base material to obtain a diamondheat sink having the fins. The thickness of the diamond heat sink shouldnaturally be at least a thickness capable of being free-standing, but ifthe heat sink is too thick, a long time is taken for the synthesisthereof to be economical, while if the heat sink is too thin, themechanical strength thereof is lowered. Thus, a thickness of 0.33 mm to5 mm preferably is used.

One embodiment of the present invention will now be described. That is,polycrystalline diamond synthesized by a gaseous synthesis method isbonded, for example by brazing, to form fins on the surface of a widelyapplicable substrate formed of a material with a relatively low heatconductivity, such as alumina. According to this structure, someimprovement can be found in the thermal property, but it is preferableto work a part of a plate-shaped base material on which a device is tobe mounted so that the fins can be embedded or buried to a depth of atleast half of the thickness of the base material. More preferably, apenetration hole is made, in which a diamond fin is buried, to providesuch a structure that heat generated by the device is released throughthe diamond having high heat conductivity, thereby increasing theradiation efficiency.

A combination of alumina and diamond, as described above, is not alwaysrequired, but use of a high heat conductivity material for the fin partis sufficient. It is preferable to use a fin material having a heatconductivity of at least 5 (W/cm·K), and polycrystalline diamondproduced by gaseous phase synthesis and having a heat conductivity of 8to 20 (W/cm·K) is the most preferable in view of the production cost andresultant properties, but high pressure synthesis single crystal diamond(15 to 30 W/cm·K) or high pressure sintered polycrystalline cubicsilicon nitride (5 to 8 W/cm·K) also can be used. The material of a basematerial to be mounted by fins can be chosen from relatively inexpensiveand low heat conductivity materials, and alumina which can be providedat a low cost and which can give actual results as a semiconductorsubstrate preferably is used. When using a material having a heatconductivity of at most 0.05 (W/cm.K), satisfactory results cannot beobtained because a temperature gradient between a fin and a part apartfrom the fin is increased. The shape of a fin should be such that thethickness is at least 50 μm for the purpose of effectively performingthermal conduction, but a thickness of more than 2 mm is not preferablebecause of the resultant cost increase. The height of a fin projectedfrom the substrate should preferably be at least 0.5 mm and effectiveradiation of heat can be achieved at a range of 2 to 15 mm.

As to the shape of a substrate to be provided with fins, penetrationworking or treatment thereof is not necessarily required, and it ispreferable that the fins reach the vicinity of semiconductor devices atleast by half of the thickness of the base material. A diamond fin canbe obtained by preparing a diamond free-standing film by an ordinarygaseous phase synthesis method and working thereof a desired shape bythe use of a laser. Moreover, high pressure synthesis single crystaldiamond can be used if it can be worked to a desired size. Any knownprocedure can be used in the gaseous synthesis of diamond.

Another embodiment of the present invention will now be described. Inthe case of using diamond as a substrate, the structure of a heat sinkpreferably is as described below. Preferably, the fins are formed fromfree-standing films of gaseous phase synthesis diamond, since such filmshave a high heat conductivity and readily are obtainable with a largearea. Any method of synthesis of this film can be used, for example,comprising growing diamond to a thickness of more than what can beobtained as a free-standing film, and then removing a base material byany of known methods (e.g. acid treatment). The free-standing film,which will be a fin part of the heat sink, is subjected to a cuttingoperation to a size corresponding to the fin, for example by use of alaser.

Since diamond having the highest heat conductivity is used as a mainbody for transporting heat of the heat sink in the present invention(when using diamond as a substrate), a high radiation effect as a wholecan be obtained, even if another material which has not such a high heatconductivity as diamond is used as a fin. The material of the fin of thediamond heat sink can be selected from those having a heat conductivityof at least 1 W/cm·K at near room temperature and resisting atemperature of 700° C. during growth of diamond. Such a materialincludes diamond, cubic boron nitride, silicon, silicon carbide,aluminum nitride, copper, tungsten, molybdenum, etc. Above all,materials having thermal expansion coefficients similar to diamondpreferably are used, selected from diamond, cubic boron nitride,silicon, silicon carbide, aluminum nitride, copper, tungsten,molybdenum, etc. Such materials also can be used as the substrate orbase material. In particular, preferably such materials are those that,when diamond is grown on an end of such substrate thereof by gaseousphase synthesis, the bonding property therebetween can be wellmaintained. Illustrative such materials are high pressure synthesissingle crystal diamond or natural single crystal diamond, gaseous phasesynthesis diamond described above, cBN sintered bodies, etc.

Considering thermal property and production efficiency, the shape of thefin preferably should be such that the thickness is 50 μm to 2 mm andthe height is at least 2 mm. The interval between fins preferably is inthe range of about 1 to 5 mm, since if too small the thermal property isremarkably degraded, while if too large the radiation area is not verylarge. This range depends on conditions of heat generation of devices tobe mounted and air cooling of a package.

Production of a diamond heat sink having fins according to the presentinvention is generally carried out by a method comprising supporting amaterial to be fins by a certain means, truing up upper ends of the finsand growing diamond thereon by a gaseous phase synthesis method. Twoexamples of the supporting method are described below in detail.

The first method comprises using blocks spaced at suitable intervals.That is, blocks made of a material stable in an atmosphere for gaseousphase synthesis of diamond are arranged with gaps therebetween intowhich fin materials are inserted. The size of the gap is suitablydetermined depending upon the interval between the fins. Base materialscapable of growing diamond by gaseous phase synthesis are spread overall the blocks. The base material should satisfy the requirement thatdiamond can be grown thereon and thereafter the base material readilycan be removed. Specifically, polycrystalline silicon can be used. It ispreferable to subject the base material to a scratching treatment so asto readily enable diamond growth.

When using diamond as a base material, the subsequent step of removingthe base material can be omitted and the base material thus can beeffectively utilized. The height of a block on which a base material isto be placed preferably should be so that the surface of the dividedbase material to be placed thereon is the same as or somewhat lower thanthe height of a fin. After the base material and fins are arranged inthis way, diamond is grown by a gaseous phase synthesis method. Thegaseous phase synthesis method of diamond is not particularly limited,but in the case of using a hot filament CVD method, for example, a heatsink with a large area can be obtained in a relatively easy manner. Thethickness to be grown can be adjusted to such an extent that afree-standing film can be obtained. After diamond is grown in a desiredthickness, the base material can if necessary be removed to obtain adiamond heat sink having fins.

The second method comprises using a base material which can be grooved.That is to say, a material is selected to be capable of satisfying suchconditions that diamond can be grown thereon, grooves can be formedtherein, and such material only readily can be removed after growth ofdiamond thereon and insertion therein of a fin material, for examplepreviously prepared diamond free-standing film at predeterminedpositions. The method of groove formation is not particularly limitedand can be performed on any one of base materials which can be worked onthe surface thereof with such a precision that the fin material can beinserted. When the fin is inserted, it is preferable that the upper endof the fin is at the same level as or is somewhat projected from thesurface of the substrate. It is not preferable that the upper end of thefin be somewhat lower than the surface of the substrate in a mannersimilar to the method using blocks. The base material used herein maybe, for example, polycrystalline silicon.

Furthermore, diamond is spread over a groove-free area of a basematerial in a manner similar to the method using blocks, and diamond isgrown thereon, whereby the step of removing the divided base materialcan be omitted and the blocks can be reused. In this case, it is alsopreferable that the relationship between the surface of the spreaddiamond and the height of the fin is that the fin is somewhat projectedfrom or is at the same level as the surface of the diamond. Thus,diamond is grown on the base material having high heat conductivity finsinserted by a gaseous phase synthesis method. In this method, ascratching treatment of the surface of the base material is alsopreferably carried out before grooving or before inserting the fins. Thediamond grown herein should have such a thickness that a free-standingfilm can be obtained. After diamond is grown to such an extent that afree-standing film is obtained, the diamond and base material are takenfrom the blocks and the base material is removed. A radiation substratehaving diamond or high heat conductivity fins on the diamond is thusobtained. As a method of removing the base material part, an acidtreatment is used.

When using the structure of a heat sink produced according to thepresent invention, there can be obtained a substrate having a higherthermal property at a relatively lower cost, compared with the prior artsubstrate structure. Accordingly, high output and high speed deviceswhich cannot be mounted on substrates using alumina can be mounted at alower cost.

Moreover, when using the structure of a diamond heat sink producedaccording to the present invention, the thermal property of the priorart package can be improved substantially and there is realized a heatsink on which high speed and high power consumption devices can bemounted. Furthermore, according to the production process of the presentinvention, a heat sink having fins using diamond, on which highperformance devices can be mounted, can be produced in a simple andeffective manner.

The following examples are given in order to illustrate the presentinvention in detail without limiting the same.

EXAMPLE 1

Diamond was synthesized at a thickness of 1 mm on a polycrystalline Sibase material (24×24×5 mm) by a hot filament CVD method under syntheticconditions of an atmosphere of methane 2%-H₂, a total pressure of 100Torr and base material temperature of 800° C. After the diamond (heatconductivity: 14 W/cm·K) was synthesized under these conditions, the Sibase material was dissolved in a mixed acid to obtain a free-standingdiamond film of 50×25×1 mm. The grown surface of this free-standing filmwas then subjected to polishing and mirror-finishing and cut into eightfilms of 6×24×1 mm using an excimer laser, after which the entiresurfaces were metallized in the order of titanium, platinum and gold.

On the other hand, as shown in FIG. 1, a heat sink part 1 (thickness: 2mm) of an ordinary alumina ceramics PGA package was subjected to a lasertreatment to form hollows 2 (FIG. 3) each having a length of 24 mm andwidth of 1 mm and was metallized in the manner similar to that describedabove. The above described diamond films, as fins 3, were inserted inrespective hollows or grooves 2 and bonded therein by gold-tin brazing.Similarly, another package was prepared with fins of high pressuresynthesis polycrystalline cubic boron nitride (heat conductivity: 6.5W/cm·K) inserted therein.

When the heat resistance of the thus obtained packages was measuredunder a state of natural air cooling, it was 20° C./W in the case of thediamond fins and 26° C./W in the case of the cubic boron nitride fins.It was thus confirmed that the heat resistance was decreased to agreater extent, compared with 35° C./W before fitting of the fins, andthe present invention was effective for improving the stability ofsemiconductor devices.

EXAMPLE 2

As shown in FIG. 5(a), a polycrystalline Si base material 1 (30×30×5 mm)was subjected to scratching by the use of diamond abrasive grains andthen eight grooves 2 22 mm in length, 1 mm in width and 4 mm in depthwere formed at intervals of 2 mm. cBN sintered bodies 3 (4×24×1 mm. 5W/cm·K) as fins were inserted into respective of the grooves, duringwhich the height of each fin was controlled by inserting a suitablespacer in the respective groove in such a manner that the upper part ofthe fin was projected above the base 1 by an amount in the range of 50to 100 μm (FIG. 5(b)). Polycrystalline diamond 4 was grown to athickness of 1 mm by a hot filament CVD method on the thus resultingpolycrystalline Si base material in which the cBN sintered bodies 3 hadbeen inserted (FIG. 5(c)). The growing conditions were an atmosphere of2% CH₄ -H₂, total pressure of 100 Torr and base material temperature of850° C. The base material Si was then removed by treating with a mixedacid to obtain a diamond heat sink having a size of 30×30×1 mm andhaving eight cBN fins 3 each having a size of 24×4×1 and embedded insubstrate 4 (FIG. 5(d)). The thus obtained heat sink had a thermalproperty of 0.8° C./W, measured under forced cooling at 2 m/sec, whichwas substantially less than the heat resistance of 3° C./W in the caseof a heat sink having no fins.

When using a Cu-W alloy (80% Cu, 20% W, 2 W/cm·K) as the fin material inthis example, the heat sink had a thermal property of 1.3° C./W.

EXAMPLE 3

Polycrystalline diamond was grown to a thickness of 600 μm by a hotfilament CVD method on a polycrystalline Si base material (25×25×5 mm)under the same conditions as those of Example 2. After dissolving thebase material, the product was subjected to processing by an excimerlaser treatment to obtain a free-standing diamond film of 2×24×0.6 mmthickness. On the other hand, a free-standing diamond film to be fins(15 W/cm·K) was similarly prepared to a thickness of 1 mm and subjectedto a laser treatment to form eight 4×24×1 mm fins 7. Then, nine Moblocks 5 (2×25×3.5 mm) were prepared and arranged at intervals of 1 mm,between which the previously prepared fins 7 were inserted. Thefree-standing diamond film of 2×24×0.6 mm thickness was divided intobase members 6 that were placed on the blocks as a first layer, as shownin FIG. 6(a), with the height of the upper surfaces of the members 6lower than that of the fins, but the different was only at most 50 μm.Diamond layer 8 was grown as a second layer to a thickness of 0.4 mmunder the same conditions as those of Example 2 on the diamond members 6arranged on the Mo blocks with diamond fins 7 sandwiched therebetween(FIG. 6(b)). After such growth, blocks 5 were removed and a diamond heatsink having a size of 26×24×1 mm and including eight diamond fins wasobtained (FIG. 6(c)), with each fin embedded in 60% of the thickness ofthe diamond substrate formed by members 6 and layer 8.

What is claimed is:
 1. A process for producing a heat sink includingfins and having an increased thermal property, said processcomprising:providing a base material with gaps; inserting fins, formedof diamond or polycrystalline cubic boron nitride and having a heatconductivity of at least 1 (W/c·K), into respective said gaps; growingdiamond on a surface of said base material and on end surfaces of saidfins, thereby forming a diamond substrate; and removing said basematerial, thus forming a heat sink including said substrate having saidfins extending therefrom.
 2. A process as claimed in claim 1, whereinsaid providing comprises forming said base material as separate membersspaced by said gaps.
 3. A process as claimed in claim 1, wherein saidproviding comprises forming said gaps as recesses or holes in said basematerial.
 4. A process as claimed in claim 1, wherein said insertingcomprises positioning inserted first end surfaces of said finssubstantially level with a first surface of said base material, suchthat after removal of said base material said first end surfaces of saidfins are substantially level.
 5. A process as claimed in claim 4,wherein said inserting further comprises positioning second end surfacesof said fins to extend beyond a second surface of said base material,such that after removal of said base material said second end surfacesof said fins are embedded in said substrate.
 6. A process as claimed inclaim 1, further comprising forming said fins by a gaseous phasesynthesis operation.
 7. A process as claimed in claim 1, furthercomprising forming a diamond or polycrystalline cubic boron nitride filmby a gaseous phase synthesis operation, and then separating said filminto plural pieces to form said fins.
 8. A process for producing a heatsink including fins and having an increased thermal property, saidprocess comprising:providing a diamond base member with gaps; insertingfins, formed of diamond or polycrystalline cubic boron nitride andhaving a heat conductivity of at least 1 (W/c·K), into respective saidgaps; growing diamond on a surface of said base member and on endsurfaces of said fins, thus forming a heat sink including a substrateformed by said diamond base member and said diamond grown thereon, withsaid fins extending from said substrate.
 9. A process as claimed inclaim 8, wherein said providing comprises forming said diamond basemember as a film by a gaseous phase synthesis operation.
 10. A processas claimed in claim 9, wherein said providing further comprisesseparating said film into plural pieces, and spacing said plural piecesto form said gaps.
 11. A process as claimed in claim 8, wherein saidproviding comprises forming said diamond base member as separate membersspaced by said gaps.
 12. A process as claimed in claim 8, wherein saidproviding comprises forming said gaps as recesses or holes in saiddiamond base member.
 13. A process as claimed in claim 8, wherein saidinserting comprises positioning inserted first end surfaces of said finssubstantially level with a first surface of said diamond base member.14. A process as claimed in claim 8, further comprising forming saidfins by a gaseous phase synthesis operation.
 15. A process as claimed inclaim 8, further comprising forming a diamond or polycrystalline cubicboron nitride film by a gaseous phase synthesis operation, and thenseparating said film into plural pieces to form said fins.
 16. A processas claimed in claim 8, further comprising positioning said diamond basemember on blocks spaced at intervals corresponding to said gaps, saidinserting comprises locating said fins in respective said intervals, andafter said growing removing said blocks.